Wireless digital networks, such as networks operating under the current Electrical and Electronics Engineers (IEEE) 802.11 standards, are spreading in their popularity and availability. Currently, the multicasting mechanism of wireless local area networks (WLANs) generally is independent of any physical layer properties.
When multicasting packets to multiple stations in a WLAN, the radio typically uses the fastest rate supported by the slowest station to determine the multicast rate at which packets will be sent. This is also referred to as multicast rate optimization. This rate may be really slow, which leads to lower multicast performance. Specifically, assuming that there are five client devices that are subscribed to the same multicast group in a WLAN. With multicast rate optimization, the system will determine which of the five client devices is associated with the slowest physical layer rate, and will use the slowest physical layer rate as the multicast rate to transmit the multicast packets to all five client devices. This is so even though the other four client devices are associated with much faster physical layer rate. Because the client devices do not send any acknowledgement frames upon receiving the multicast packets, sending the multicast packets at the slowest physical layer rate increases the likelihood that all five client devices will receive the multicast packets successfully.
Here, physical layer rate generally refers to the actual rate at which the WLAN radio is modulated to transmit packets. Note that, the actual time required to complete transmission of multicast packets also depends on how frequently the packets are being transmitted. Frequent transmission and fast physical layer rate will reduce the amount of transmission time required for multicast packet transmissions, and thus improve the overall data transmission rate. The physical layer rate can also be referred to as modulation and coding scheme (MCS) rate.
Alternatively, the system can convert a multicast packet to multiple unicast packets destined to various client devices. The multicast-to-unicast conversion may be applicable only when there are a small number of client devices in the WLAN. Therefore, an enhanced mechanism (other than physical layer rate optimization mechanism and/or multicast-to-unicast conversion mechanism) is needed to achieve true multicast high performance in WLANs with a large number of client devices or when multicast-unicast conversion is not generally used.